In industrial measurements technology, especially also in connection with the control and monitoring of automated manufacturing processes, for ascertaining characteristic measured variables of media, for example, liquids and/or gases, flowing in a process line, for example, a pipeline, often such measuring systems are used, which, by means of a measuring transducer of vibration-type and a transmitter electronics connected thereto and most often accommodated in a separate, electronics housing, induce reaction forces in the flowing medium, for example, Coriolis forces, and produce, repetitively derived from these, measurement values correspondingly representing the at least one measured variable, for example, a mass flow rate, a density, a viscosity or some other process parameter. Such measuring systems—often formed by means of an In-line measuring device in compact construction with integrated measuring transducer, such as, for instance, a Coriolis mass flow meter,—are long since known and have proven themselves in industrial use. Examples of such measuring systems having a measuring transducer of vibration-type or also individual components thereof, are described e.g. in EP-A 317 340, JP-A 8-136311, JP-A 9-015015, US-A 200710119264, US-A 2007/0119265, US-A 2007/0151370, US-A 2007/0151371, US-A 2007/0186685, US-A 2008/0034893, US-A 200810141789, U.S. Pat. Nos. 4,680,974, 4,738,144, 4,777,833, 4,801,897, 4,823,614, 4,879,911, 5,009,109, 5,024,104, 5,050,439, 5,291,792, 5,359,881, 5,398,554, 5,476,013, 5,531,126, 5,602,345, 5,691,485, 5,734,112, 5,796,010, 5,796,011, 5,796,012, 5,804,741, 5,861,561, 5,869,770, 5,945,609, 5,979,246, 6,047,457, 6,092,429, 6,073,495, 6,311,136, 6,223,605, 6,330,832, 6,397,685, 6,513,393, 6,557,422, 6,651,513, 6,666,098, 6,691,583, 6,840,109, 6,868,740, 6,883,387, 7,017,424, 7,040,179, 7,073,396, 7,077,014, 7,080,564, 7,134,348, 7,216,550, 7,299,699, 7,305,892, 7,360,451, 7,392,709, 7,406,878, WO-A 00/14 485, WO-A 01/02 816, WO-A 2004/072588, WO-A 2008/013545, WO-A 2008/07 7574, WO-A 95/29386, WO-A 95/16897 or WO-A 99 40 394. Each of the therein illustrated measuring transducers comprises at least one, essentially straight, or curved, measuring tube accommodated in a measuring transducer housing and conveying, or guiding, the, in given cases, also extremely rapidly, or extremely slowly, flowing, medium. In operation of the measuring system, the at least one measuring tube is caused to vibrate for the purpose of generating oscillation forms influenced by the medium flowing through the measuring tube.
In the case of measuring transducers having two measuring tubes, these are most often integrated into the process line via a flow divider extending on the inlet side between the measuring tubes and an inlet-side connecting flange as well a via a flow divider extending on the outlet side between the measuring tubes and an outlet-side connecting flange. In the case of measuring transducers having a single measuring tube, the latter communicates with the process line most often via an essentially straight connecting tube piece opening on the inlet side, as well as via an essentially straight connecting tube piece opening on the outlet side. Additionally, each of the illustrated measuring transducers having a single measuring tube comprises, in each case, at least one one-piece or multipart, for example, tube-, box- or plate-shaped, counteroscillator, which is coupled to the measuring tube on the inlet side for forming a first coupling zone and which is coupled to the measuring tube on the outlet side for forming a second coupling zone, and which, during operation, essentially rests or oscillates opposite-equally to the measuring tube, thus with equal frequency and opposite phase. The inner part of the measuring transducer formed by means of measuring tube and counteroscillator is most often held, especially in a manner enabling oscillations of the inner part relative to the measuring tube, in a protective measuring transducer housing alone by means of the two connecting tube pieces, via which the measuring tube communicates during operation with the process line. In the case of the measuring transducers, for example, as illustrated in U.S. Pat. Nos. 5,291,792, 5,796,010, 5,945,609, 7,077,014, US-A 2007/0119264, WO-A 01 02 816 or also WO-A 99 40 394, having a single, essentially straight, measuring tube, the latter and the counteroscillator are, as in the case of conventional measuring transducers quite usual, oriented essentially coaxially relative to one another. In the case of usually marketed measuring transducers of the aforementioned type, most often also the counteroscillator is essentially tubular and embodied as an essentially straight, hollow cylinder, which is so arranged in the measuring transducer, that the measuring tube is at least partially jacketed by the counteroscillator. Most often used as materials for such counteroscillators, especially also in the case of application of titanium, tantalum or zirconium for the measuring tube, are comparatively cost effective steel types, such as, for instance, structural steel or free-machining steel.
Selected as excited oscillation form—the so-called wanted mode—in the case of measuring transducers having curved, e.g. U, V- or Ω-like formed, measuring tubes is usually that eigenoscillation form, in the case of which the measuring tube moves in a pendulum-like manner at least partially in a lowest natural resonance frequency about an imaginary longitudinal axis of the measuring transducer, like a cantilever clamped on one end, whereby Coriolis forces are induced in the through flowing medium dependent on the mass flow. These forces, in turn, lead to the fact that superimposed on the excited oscillations of the wanted mode, in the case of curved measuring tubes, thus pendulum-like, cantilever oscillations, are thereto equal-frequency, bending oscillations according to at least one, likewise natural, second oscillation form, the so-called Coriolis mode. In the case of measuring transducers with curved measuring tube, these cantilever oscillations in the Coriolis mode caused by Coriolis forces usually correspond to that eigenoscillation form, in the case of which the measuring tube also executes rotary oscillations about an imaginary vertical axis directed perpendicular to the longitudinal axis. In the case of measuring transducers with straight measuring tube, in contrast, for the purpose of producing of mass flow dependent Coriolis forces, often such a wanted mode is selected, in the case of which the measuring tube executes, at least partially, bending oscillations essentially in a single imaginary plane of oscillation, such that the oscillations in the Coriolis mode are bending oscillations of equal oscillation frequency coplanar with the wanted mode oscillations. Due to the superpositioning of wanted- and Coriolis modes, the oscillations of the vibrating measuring tube registered by means of the sensor arrangement on the inlet side and on the outlet side have a measurable phase difference also dependent on the mass flow. Usually, the measuring tubes of such measuring transducers, applied e.g. in Coriolis mass flow meters, are excited during operation to an instantaneous natural resonance frequency of the oscillation form selected for the wanted mode, especially with oscillation amplitude controlled to be constant. Since this resonance frequency is dependent, especially, also on the instantaneous density of the medium, supplementally also the density of flowing media can be measured by means of market-usual Coriolis mass flow meters, in addition to the mass flow. Additionally, it is also possible, as, for example, shown in U.S. Pat. Nos. 6,651,513 or 7,080,564, directly to measure, by means of measuring transducers of vibration-type, the viscosity of the through flowing medium, for example, based on an exciter energy or excitation power required for maintaining the oscillations, and/or based on a damping of oscillations (especially those in the aforementioned wanted mode) of the at least one measuring tube resulting from a dissipation of oscillatory energy. Moreover, also other measured variables derived from the aforementioned primary measured values of mass flow rate, density and viscosity can be ascertained, such as, for instance, the Reynolds number; compare U.S. Pat. No. 6,513,393.
For exciting oscillations of the at least one measuring tube, measuring transducers of vibration-type have, additionally, an exciter mechanism driven during operation by an electrical driver signal, e.g. a controlled electrical current, generated and correspondingly conditioned by the mentioned driver electronics. The exciter mechanism excites the measuring tube to bending oscillations in the wanted mode by means of at least one electro-mechanical, especially electro-dynamic, oscillation exciter acting practically directly on the measuring tube and flowed through during operation by an electrical current. Furthermore, such measuring transducers comprise a sensor arrangement having oscillation sensors, especially electro-dynamic oscillation sensors, for the at least pointwise registering of inlet-side and outlet-side oscillations of the at least one measuring tube, especially those in the Coriolis mode, and for producing electrical sensor signals influenced by the process parameter to be registered, such as, for instance, the mass flow or the density, and serving as primary signals of the measuring transducer. As, for example, described in U.S. Pat. No. 7,216,550, in the case of measuring transducers of the type being discussed, in given cases, also the oscillation exciter can at least at times be used as oscillation sensor and/or an oscillation sensor at least at times can be used as oscillation exciter. The exciter mechanism of measuring transducers of the type being discussed includes, usually, at least one electrodynamic oscillation exciter and/or an oscillation exciter acting differentially on the at least one measuring tube and the, in given cases, present counteroscillator or the, in given cases, present, other measuring tube, while the sensor arrangement comprises an inlet-side, most often likewise electrodynamic, oscillation sensor as well as at least one outlet-side oscillation sensor constructed essentially equally thereto. Such electrodynamic and/or differential oscillation exciters of usually marketed measuring transducers of vibration-type are formed by means of a magnet coil, through which an electrical current flows, at least at times. In the case of measuring transducers having a measuring tube and a thereto coupled counteroscillator, most often the magnet coil is affixed to the latter. Such oscillation exciters further include a rather elongated, especially rod-shaped, permanent magnet interacting with the at least one magnet coil, especially plunging into it, and serving as armature and affixed correspondingly to the measuring tube to be moved. The permanent magnet and the magnet coil serving as exciter coil are, in such case, usually so oriented, that they extend essentially coaxially relative to one another. Additionally, in the case of conventional measuring transducers, the exciter mechanism is usually embodied in such a manner and so placed in the measuring transducer, that it acts essentially centrally on the at least one measuring tube. In such case, the oscillation exciter (and, insofar, the exciter mechanism) is, such as, for example, also shown in the case of the measuring transducers proposed in U.S. Pat. Nos. 5,796,010, 6,840,109, 7,077,014 or 7,017,424, most often affixed at least pointwise along an imaginary central, peripheral line of the measuring tube outwardly thereon. Alternatively to an exciter mechanism formed by means of oscillation exciters acting rather centrally and directly on the measuring tube, as, among other things, provided in U.S. Pat. No. 6,557,422, 6,092,429 or 4,823,614, for example, also exciter mechanisms formed by means of two oscillation exciters affixed not in the center of the measuring tube, but, instead, rather at the inlet and outlet sides, respectively, thereof can be used, or, as, among other things, provided in U.S. Pat. Nos. 6,223,605 or 5,531,126, for example, also exciter mechanisms formed by means of an oscillation exciter acting between the, in given cases, present counteroscillator and the measuring transducer housing can be used. In the case of most market-usual measuring transducers of vibration-type, the oscillation sensors of the sensor arrangement are, as already indicated, at least, insofar as they work according to the same principle of action, embodied essentially of equal construction as the at least one oscillation exciter. Accordingly, also the oscillation sensors of such a sensor arrangement are most often formed, in each case, by means of at least one magnet coil—usually affixed to the, in given cases, present counteroscillator—, at least at times passed through by a variable magnetic field and, associated therewith, at least at times supplied with an induced measurement voltage, as well as by means of a permanently magnetic armature, which delivers the magnetic field. The armature is affixed to the measuring tube and interacts with the at least one coil. Each of the aforementioned coils is additionally connected by means of at least one pair of electrical connecting lines with the mentioned transmitter electronics of the in-line measuring device. The connecting lines are led most often on as short as possible paths from the coils via the counteroscillator to the measuring transducer housing.
As, among other things, discussed in the initially mentioned U.S. Pat. Nos. 7,406,878, 7,305,892, 7,134,348, 6,513,393, 5,861,561, 5,359,881 or. WO-A 2004/072588, a further parameter quite relevant for the operation of the measuring system as such and/or for the operation of the plant, in which the measuring system is installed, can be a pressure loss in the flow—, for example, a pressure loss caused by the measuring transducer and, insofar, by the measuring system. Pressure loss in the flow is important, especially, also for the case, in which the medium has two- or more phases, for instance, a liquid gas mixture, and/or in which one must contend with, or necessarily prevent, during operation, undesired cavitation as a result a subceeding, or falling beneath, of a minimum static pressure in the flowing medium. In the case of the measuring systems illustrated in U.S. Pat. Nos. 5,359,881 or 7,406,878, a pressure drop across the measuring transducer during operation is, for example, ascertained by the features that, at a first pressure measuring point in the inlet region of the measuring transducer, or directly upstream therefrom, a first static pressure in the flowing medium is registered by means of a first pressure sensor, and, at a second pressure measuring point in the outlet region of the measuring transducer, or directly downstream therefrom, a second static pressure in the flowing medium is registered by means of an additional, second pressure sensor, and, by means of hydraulic pressure measuring mechanism and/or by means of the respective transmitter electronics, these are repetitively converted into a corresponding pressure difference, measured value. In U.S. Pat. Nos. 7,305,892, or 7,134,348, there is additionally described a method executable by means of a measuring transducer of vibration-type for measuring a pressure difference, in the case of which, on the basis of an oscillatory response of the at least one measuring tube to a multimodal oscillation excitation, as well as on the basis of physical-mathematical models furnished in the transmitter electronics for the dynamics of the measuring system (formed here as a Coriolis, mass flow measuring device), a pressure, or pressure drop, in the medium flowing through the measuring transducer is ascertained.
A disadvantage of the solutions known from the state of the art for pressure measurement, especially also for pressure difference measuring by means of measuring transducer of vibration-type, is, however, to be seen in the fact that either correspondingly modified exciter mechanisms and/or correspondingly modified driver electronics need to be used, or, however, additional pressure sensors provided. Associated therewith, both the design complexity of the measuring system as well as also the experimental effort in the calibrating of such measuring systems increase in extreme measure, since the foundational physical mathematical models for the pressure—, or the pressure difference, measuring, for the purpose of achieving a high accuracy of measurement, are very complex, and have, associated therewith, a large number of coefficients, which need to be supplementally calibrated, in given cases, also in the course of a wet-calibration performed first on-site at the installed measuring system.